Advantages

• Rapid onset of action, often within minutes, because of rich blood supply.
• Avoids first-pass hepatic metabolism, increasing bioavailability for drugs extensively metabolized by the liver, such as nitroglycerin. NCBI route overview
• Avoids degradation in the gastrointestinal tract by acid or digestive enzymes.
• Non-invasive, convenient, and painless. No sterile equipment or trained person is needed.
• Useful when swallowing is difficult or when a rapid effect is needed without an injection.
• Medication can sometimes be removed from the mouth if an adverse effect occurs before complete absorption.

Disadvantages

• Only small doses of potent drugs can be given, because the area available for absorption is limited.
• The drug must be lipid-soluble and able to cross oral mucosa. Many drugs are unsuitable.
• Absorption may be variable, affected by saliva, mucosal condition, smoking, food or drink, and whether the patient accidentally swallows the drug.
• Not suitable for patients who are unconscious, uncooperative, vomiting, very dry-mouthed, or unable to keep the drug under the tongue.
• May cause local irritation, unpleasant taste, or mucosal ulceration.
• Patients must avoid chewing, swallowing, eating, or drinking until the tablet dissolves, which can reduce adherence.
• It is generally unsuitable for sustained large-dose therapy.